1. Field of the Invention
This invention relates generally to data communication systems for guided boring and drilling apparatus, a data acquisition and data link system for any information, uphole or downhole, and more particularly to a wireless downhole electromagnetic data transmission system utilizing microprocessor controlled frequency synthesis for two-way communication between the surface and a downhole guided boring or drilling apparatus wherein the system selects one or more frequencies in the range from 15 Hz to 100 kHz having an optimum signal-to-noise ratio at a given transmitter power level and baud rate.
2. Brief Description of the Prior Art
Since its inception into the underground utility construction industry, guided boring technology has experienced rapid advances and evolution in the types of systems available and the range and accuracies that can be achieved. Guided boring requires the capability to control hole direction and monitor its position in space. Currently, most small guided boring systems such as rod pushers, wet bore, and compaction systems utilize "pipe locators" to track and orient the boring head. These "pipe locators" consist of a small active transmitter placed near the drill head and a pair of highly tuned receiver coils. The devices are low cost and provide reasonably accurate data. Their main limitations are the need for surface access and shallow depth capability. Conventional "pipe locator-based tracking systems" have become more difficult and impractical to employ in areas with difficult access and at increased depths. In addition, the risk of depending on the interpretive nature of "pipe-locators" and the incomplete information they provide grows less acceptable in direct relation to overall job costs.
Solid-state compasses known as "steering tools" are used for tracking and guidance of boring tools when economics and work conditions allow. These "steering tools" are significantly more expensive than "pipe locators" and require an insulated wire connecting the downhole instrumentation to the surface.
More recently, wireless systems, referred to generically as "Measurements-While-Drilling (MWD) systems have been developed. The MWD systems provide wellbore directional data and/or formation without requiring an insulated hard-wire link to bring information to the surface. The wireless, "Measurements-While-Drilling" (MWD) systems provide higher reliability, simpler operation, higher speed, greater directional control, longer distances; and accommodate a greater range of hole sizes. The cost savings, minimum surface disruption, miniaturization, and ability to provide the drilling contractor with real-time information of bottom-hole conditions, has made MWD technology especially useful in the utilities industry.
Electromagnetic systems operating under crystal controlled frequency generators are known which operate with one (pulse-width modulation) or two (frequency shift keying (FSK) fixed signals of low frequency, typically less than 25 hertz. These low frequencies are used because of the reduction in signal attenuation with decreasing frequency. A substantial limitation of the prior art electromagnetic systems is the maximum data rate that can be achieved and their nonadaptive nature relative to avoiding interference in the capture frequency band being used.
Directional boring systems require a physical means to change the direction of hole travel in a predictable manner and a method for tracking hole position in space. The position and direction of the drill bit is fully specified in six degrees of freedom: x, y and z coordinates, azimuth, inclination and tool face. In practice, the location and direction instrumentation measuring systems do not make direct measurements of all of these variables. Rather, a sufficient number of readings are made which combined with other data, such as the amount of pipe in the hole, can be used to calculate the remaining variables. For example, "pipe-locators" provide depth, plan location and tool face. Azimuth and inclination must be found by interpolation between the survey points. The accuracy of this interpolation depends on how closely spaced the readings are taken relative to the actual tool path. Similarly, "steering tools" and other compass-type systems measure the three rotation angles in drilling. This information is combined with drilled distance to compute the x, y, and z coordinates.
It is important to understand the differences in capabilities and operating requirements between guidance systems used in "oil and gas well boring" (energy exploration) and those used in "utility boring". With respect to accuracy, "utility boring" requires greater resolution and more frequent surveys to maintain the proper path. This is due in large part to the shallow depths and highly congested environment in which they operate. With respect to packaging, "utility boring" systems must be shorter in length and usually smaller in diameter. Due to the lower values of pressure, temperature, torque, and bit weight in "utility boring" the problems associated with downsizing are more easily overcome than in "oil and gas well boring". It is also important to note that the utility market cannot bear the daily rental rates or purchase costs normally required by oil and gas directional boring service companies.
The use of electromagnetic telemetry systems is well documented. Rubin (U.S. Pat No. 4,725,837) discloses the use of a toroidal coupled telemetry system in which the secondary winding comprises a plurality of turns wrapped around a generally annular core member. The present invention differs from Rubin in that the digital data, transmitted with a frequency shift keying encoding scheme, is injected directly into the drill string--earth conduction path with no reliance on coil induction. This results in a substantially simpler design that is easier and lower in cost to produce.
Further, Rubin et al., (U.S. Pat. No. 4,691,203) describe use of an insulated gap sub composed of conductive sleeves heat shrunk onto an insulated sleeve and a central mandrel. This differs from the present invention which obtains the required insulating qualities without sacrificing strength of the outside collar that must transmit torque and axial loads developed during the drilling process. The current invention places a split, conductive ring over the mandrel in a keyed arrangement. A coating, such as polyurethane is placed between the two elements to achieve electrical insulation. Unlike Rubin et al., the coating is not required to transmit energy--resulting in a mechanically more robust design.
Van Steenwyck (U.S. Pat. No. 5,130,706) describes an apparatus comprising a direct switching element (e.g., magnetic reed switch) for coupling transmission energy from a downhole energy source to the earth-drill string system. The switching element can be used to control the time duration, wave shape or frequency of the output energy to be transmitted. By contrast, the present invention uses a frequency synthesizer to encode the telemetered data in a FSK format. The synthesizer uses countdown registers to generate two selected frequencies. One frequency is transmitted to represent a logic one and the other frequency is transmitted to represent a logic zero. The logic level signals are used to control transistors which switch the battery pack on and off.
The present invention has been under development for some time and an early experimental version was presented at the 1992 NO-DIG International Conference in Washington, D.C. on Apr. 20-24, 1992, and a paper entities ACCUNAV.TM., REMOTE GUIDANCE FOR DIRECTIONAL BORING was presented describing this early development work.
The present invention is distinguished over the prior art in general, and these patents in particular by a wireless downhole electromagnetic data transmission system and method utilizing microprocessor controlled frequency synthesis for two-way communication between the surface and a downhole guided boring or drilling apparatus in the range of from 100 Hz to 100 kHz. A nonmagnetic downhole probe unit connected between a drill motor or drill bit and the drill string contains data gathering and transmission components including accelerometers which measure the earth's gravity vector and fluxgate magnetometers which read the earth's magnetic field and serve as power line proximity sensors. A non-magnetic housing, however, may be used in applications where no magnetometer is being used. The apparatus may also be used in near-bit measurements. The drill pipe acts as an electrical lossy, single conductor with the earth forming the electrical return path. Sensory data gathered by the downhole probe is encoded in digital format and impressed upon the drill string using frequency shift keying of the electromagnetic energy waves and is picked off at the surface by a signal receiver-demodulator and message processor unit. The surface unit instructs the downhole probe to transmit multiple frequencies and selects one or more frequencies with the most favorable signal-to-noise ratio(s) in response to local conditions to maximize the transmission distance at a selective frequency band range and given transmitter power level and baud rate. The received signal is filtered, demodulated, processed and displayed at the surface and gravity and magnetic field vectors are combined with the created hole length to calculate x, y, and z hole coordinates and derive hole position vectors.